A high pressure fuel pump typically has a head with a turret from which axially protrudes a piston reciprocally moving inside a bore of the head. The fuel pressurized is hot and some of it leaks along the piston and exits at the extremity of the turret. Fluid shear losses heat the fuel further in the clearance between the piston and bore, so the exit temperature is extremely high. An important thermal gradient exists across the turret and consequently the piston thermal expansion exceeds the turret thermal expansion diminishing the clearance and potentially damaging the pump. To avoid such problem the bore-piston clearance is typically sized to accommodate this difference however this enables more fuel leakage and therefore lower pump efficiency.